[Elecraft] (no subject)

[Don Brown]

Hi

OK here are the results of a little playing around with the Ohmite =
resistors. I had a few 51 ohm versions around. I also had a heatsink and =
fan for a Pentium III type computer CPU. You Know the type that clamp on =
top of the CPU chips with the little 12 volt fan attached. First I =
connected the resistor to my MFJ antenna analyzer and it had a 1:1.1 SWR =
to the limits of the analyzer (170 MHz) then I connected it to the Autek =
RF5 VHF analyzer and went from 30 MHZ to 549 MHZ and 1:1.0 all the way =
up. As I only had 51 ohm resistors I used DC to test the dissipation. I =
connected two in parallel on the heatsink and connected up a power =
supply. I could run up to 25-30 watts with out running the fan and the =
heatsink only got a little warm. Then I ran the power up to 60 watts and =
connected a 9 volt battery to the fan. After about 15 minutes the =
heatsink was about 60 degrees C. Running the fan at 12-14 volts should =
bring it down more. I will do some more testing.=20

It looks like if you used one of the bigger Pentium 4 type heatsinks and =
3 150 ohm resistors in parallel you could make a 100 watt dummy load =
that would do several hundred watts for a short time with the fan =
running at 12 volts. The whole thing could be mounted in a small box =
with appropriate holes for air flow. The fan could run from your power =
supply or you could install a 9 volt battery or 2 to run the fan when =
required. I like the idea of an air cooled dummy load better than the =
oil cooled. The oil seems to always leak and get all over.

These heatsinks are available for around $15-$20 with a fan from =
computer suppliers like JDR Microdevices. Or if you can find an old =
computer motherboard that a computer upgrade shop is throwing away you =
can rob one off of it maybe for the asking.

Don Brown
KD5NDB
  ----- Original Message -----=20
  From: Tom Hammond=20
  To: Don Wilhelm ; Don Brown ; Elecraft=20
  Sent: Wednesday, August 06, 2003 8:20 AM
  Subject: Re: [Elecraft] (no subject)


  Man folks, you took the words right out of my mouth... I've been doing =
a=20
  little research on this very thing for economical dummyloads.

  I've been considering two (2) of the 100 Ohm 35W thick film resistors =
in=20
  parallel.

  My one concern has been the accuracy of the resistance however. If I=20
  recall, the specs rate than at 5%, which shouldn't be at all bad. And =
if I=20
  parallel two, I may have a better chance of 'swamping' (averaging) out =
any=20
  single-device error.

  If someone beats me to the punch, PLEASE post your results to the=20
  reflector... this loots to be such an easy AND ECONOMICAL project that =
no=20
  one (and YOU KNOW WHO YOU ARE - <G>) could grouse about not being able =
to=20
  afford a dummyload for their K2, and for their K2/100 for that matter =
(at=20
  least for short periods of time.

  Let's go fer it, guys... it looks to be too good to pass up.

  73,


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